Materials Science and Engineering A334 (2002) 6 – 10 Simulation of -hydride precipitation in bi-crystalline zirconium under uniformly applied load X.Q. Ma a , S.Q. Shi a, *, C.H. Woo a , L.Q. Chen b a Department of Mechanical Engineering, The Hong Kong Polytechnic Uniersity, Hung Hom, Kowloon, Hong Kong b Department of Materials Science and Engineering, Pennsylania State Uniersity, Uniersity Park, PA 16802, USA Received 11 June 2001 Abstract The morphology evolution of -hydride precipitation and growth in a zirconium bi-crystal was simulated using a phase field kinetic model. The effects of grain boundary and uniformly applied load were studied. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg – Landau equations for the structural variables and the Cahn – Hilliard diffusion equation for the concentration variable. It is demonstrated that nucleation density of the hydride at the grain boundary increases as compared to the bulk and favorable hydride precipitation at the grain boundary become weaker when an external load is applied. The result also showed that hydrides will grow in those habit planes that are near the perpendicular direction of the applied tensile load. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Computer simulation; Zirconium hydride; Bi-crystal; Phase-field kinetic model; Morphology evolution www.elsevier.com/locate/msea 1. Introduction The mechanical properties of most materials strongly depend on their phase composition and distribution. Phase transformation often causes the change of their properties. The phase-field kinetic model is an effective model in describing the morphological evolution during phase transformation. It has been successfully applied to study the morphology of the second phase precipita- tion in many materials [1–5]. In this work, -hydride precipitation in a bi-crystal zirconium with and without uniformly applied tensile load is studied. Zirconium is a structural material in nuclear power industry owing to its combination of good mechanical properties, excellent corrosion resis- tance and low neutron absorption cross-section. How- ever, it often works under hot water environment. Zirconium will gradually pick up hydrogen from the hot water environment during service. When hydrogen concentration reaches a certain level, zirconium hydride will form. Since the brittleness of the hydride, the mechanical properties of the material will degrade, and fracture may initiate at the hydrides. It is believed that the critical conditions for fracture initiation at hydrides are controlled by the morphology and microstructure of hydride precipitates in zirconium [6 – 9]. The morphology of -hydride in zirconium and zirco- nium alloy were studied by Baily [10], Roy and Jacqes [11] and Bradbrook [12] et al. using TEM. They ob- served a habit plane of {101 – 0} type in zirconium and needle-like hydrides growing in 112 – 0 directions. Fa- vorable precipitation of -hydride at the grain boundary in polycrystalline zirconium was also re- ported [10]. It has been observed that stress plays an important role in the morphology of hydrides. Effect of tensile stress on hydride orientation has been studied experimentally by Walter [13], Bayark [14] and Marchel [15] et al. The results show that in zirconium, the tensile stress tend to change the habit plane in such fashion that the hydrides orient perpendicular to the tensile stress and that some critical stress might be necessary in order for the hydride to reorient itself. However it can be very expensive to study experi- mentally the morphological evolution of hydride pre- cipitates in irradiated materials because of the cost involved for irradiation protection. Unfortunately, such * Corresponding author. Tel.: +852-2766-7821; fax: +852-2365- 4703. E-mail address: mmsqshi@polyu.edu.hk (S.Q. Shi). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5093(01)01770-1